Conventionally, with an object of protection of privacy and prevention of illegal leakage of actual address, in the electronic mail communication, there is a mail delivery system that gives an alias address different from an actual address to a user and relays an alias mail addressed to an alias address to transfer the alias mail to the actual address. Specifically, in an alias mail relay server including a DB (database) in which alias addresses and actual addresses are associated with each other, when the alias mail relay server receives an alias mail having an alias address as a destination, the alias mail relay server acquires an actual address corresponding to the alias address with reference to the DB and replaces the destination of the alias mail with the actual address to transfer the alias mail to the actual address.
On the other hand, the alias mail relay server has problems explained below. For example, when a mailing list in which a user registers an alias address is a mailing list for delivering only a mail posted from the registered address (the alias address) to other users, a transmission source of an outgoing mail of the user is an actual address. As a result, the user cannot post the outgoing mail to the mailing list. When the user uses the alias address to conceal the actual address, if the user replies to an alias mail transferred from the alias mail relay server, a transmission source of the reply mail is the actual address. As a result, the user cannot return a mail.
Therefore, there is also a conventional technology for receiving a mail having an actual address as a transmission source address and replacing the actual address with an alias address as this transmission source to transfer the mail. For example, in a Patent Document 1 (Japanese Patent Application Laid-Open No. 2002-152246), in a transfer apparatus including a DB (database) in which alias addresses and actual addresses are associated with each other, when the transfer apparatus receives a mail having an actual address as a transmission source, the transfer apparatus acquires an alias address corresponding to the actual address with reference to the DB and replaces the actual address with the alias address as the transmission source of the mail to transfer the mail.
In addition to the conventional technology described above, there is also a mail delivery system that allows a user to freely generate a plurality of alias addresses and acquire an actual address without using a DB in which alias addresses and actual addresses are associated with each other (see, for example, a Nonpatent Literature 1). Specifically, the user freely generates a large number of alias addresses obtained by adding an arbitrary control character string (ToTanaka) after a character string (suzuki) for identifying the user such as suzuki.ToTanaka@isp-A.ne.jp and uses the alias addresses according to purposes. In an alias mail relay server, when an alias mail having an alias address as a destination is received, the alias mail relay server acquires an actual address from the character string (e.g., suzuki) for identifying the user in the alias address and replaces the destination of the alias mail with the actual address to transfer the alias mail.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-152246
Nonpatent Literature 1: Taiga Aoki, “‘Spam Slicer’, a service for rooting up spam mails, has been started in the United States”, Oct. 15, 2003.